A unique presentation of metaplastic breast carcinoma, NOS: A case report.

Key Clinical Message: A rare form of invasive breast carcinoma, NOS, also known as matrix‐producing carcinoma made up of epithelial and mesenchymal components. Usually, they are triple negative and clinically aggressive and respond poorly to neoadjuvant systemic therapy. Metaplastic breast carcinomas (MBCs) are ductal carcinomas that undergo metaplasia to form nonglandular growth patterns. They are extremely rare, constituting less than 1% of all invasive breast carcinomas. Matrix‐producing carcinoma is an exceedingly rare form of MBC distinguished by a ductal carcinomatous component with direct transition to areas of cartilaginous or osseous differentiation without the presence of an intervening spindle cell element. MBCs are clinically aggressive, but matrix‐producing subtypes have a relatively better prognosis. The tumors are usually triple negative. Therefore, surgery and chemotherapy are the main therapeutic approaches. Our report describes this unique form of MBC with prominent osseous differentiation in a 33‐year‐old female patient. Its distinct histological features and peculiar clinical behavior necessitate a thorough understanding of this one‐of‐a‐kind disease entity. [ABSTRACT FROM AUTHOR]